WASHINGTON (Reuters) - U.S. Senator Bob Corker was notably lukewarm on Sunday in his endorsement of the fellow Republican running for his seat in Tennessee in November, saying he was supporting her but would not campaign against her Democratic opponent.

Asked why Tennessee voters should choose Republican U.S. Representative Marsha Blackburn over Democrat Phil Bredesen, Corker said: "Well, I think most people in our state - it is a red state - will focus on the first vote she makes. And that is the vote to elect the majority leader.

"I think, at the end of the day, that is going to be a big factor in the race," Corker, who is not seeking re-election, said on CNN's "State of the Union."

Pressed on why he was not giving a more robust endorsement, Corker said after a considerable pause: "I'm supporting the nominee. I have worked with the nominee for some time. And I don't know what else to say."

Republicans are trying to retain control of the 100-member U.S. Senate, where they have a 51-49 advantage, in November congressional elections.

The Tennessee contest is expected to be a tight race in a state that leans heavily Republican. President Donald Trump won Tennessee in the 2016 presidential election by 26 percentage points.

Corker, chairman of the Senate Foreign Relations Committee and a frequent critic of Trump, swatted away a question about a Washington Post report that Senate Majority Leader Mitch McConnell admonished him last week for having praised Bredesen, a former Tennessee governor, whom Corker described as a longtime friend.

"I'm supporting the nominee. I've sent the maximum check, plan to vote for them," Corker said on ABC's "This Week."

But Corker said Bredesen was a friend of his and he was "not going to campaign against him."